Prior art transport refrigeration systems commonly control the temperature of a served space by cooling and hot gas heating modes above and below a predetermined set point temperature, respectively. A high speed relay is operable at predetermined temperatures above and below set point, for controlling when the compressor speed is changed between two values which are commonly called high speed and low speed.
During the cooling mode, the output of the compressor is directed via an appropriate valve arrangement, such as a three-way valve, or two separate valves, through a refrigeration circuit which includes a condenser, receiver, expansion valve, evaporator, and usually an accumulator. During a heating mode, which includes heating cycles for controlling set point, as well as heating cycles for defrost purposes, the hot gas output of the compressor is directed via the valve arrangement through a refrigeration circuit which includes only the evaporator and accumulator. The load on the prime mover reflects evaporator pressure during the cooling mode, and it reflects compressor gas discharge pressure during the heating mode. The expansion valve, which limits suction pressure during a cooling mode, is by-passed during a heating mode, and is thus not available for limiting suction pressure. A suction pressure throttling valve, also called a crankcase pressure regulator, is usually provided in the suction line to regulate the amount of refrigerant returning to the compressor, which in turn limits the suction pressure and the load on the prime mover during the heating mode of the system.
The throttling valve has the disadvantage of always being in an active refrigeration circuit, including during the cooling mode when higher compressor discharge pressures can be tolerated without overloading the prime mover. The pressure reduction caused by the throttling valve during a cooling mode thus limits cooling capacity, and it adds significant cost to the system. Also, throttle valves having a rating suitable for use with large capacity six cylinder compressors provide an unusually high and thus undesirable pressure drop. Providing a high pressure cutout device responsive to head pressure to terminate a heating mode could be used instead of a suction pressure throttling valve, but it causes needless cycling of the system between modes.
The high speed cooling and high speed heating modes of prior art transport refrigeration systems are normally required for the purposes of: (a) providing a fast temperature pull-down in the served space upon initial start-up of the system, (b) increasing heating capacity to prevent freezing of a perishable load in the event of low ambient temperatures, and (c) for providing a fast defrost cycle. High speed operation increases fuel consumption and load on the prime mover, however, and should be avoided in order to increase system efficiency, unless there is no viable alternative.
Thus, it would be desirable to provide a new and improved method of operating a transport refrigeration system having a six cylinder compressor which provides maximum cooling capacity without the danger of overloading the compressor and prime mover during a heating or defrost cycle, which limits prime mover horsepower by always operating with the fewest possible number of loaded compressor cylinders in any given heating or cooling mode, and which limits the amount of time the system spends at high speed.